Optical and magnetic measurements of horseradish peroxidase. 3. Electron paramagnetic resonance studies at liquid-hydrogen and -helium temperatures

In Vitro Heme Coordination of a Dye-Decolorizing Peroxidase-The Interplay of Key Amino Acids, pH, Buffer and Glycerol

Dye-decolorizing peroxidases (DyPs) have gained interest for their ability to oxidize anthraquinone-derived dyes and lignin model compounds. Spectroscopic techniques, such as electron paramagnetic resonance and optical absorption spectroscopy, provide main tools to study how the enzymatic function is linked to the heme-pocket architecture, provided the experimental conditions are carefully chosen. Here, these techniques are used to investigate the effect of active site perturbations on the structure of ferric P-class DyP from Klebsiella pneumoniae (KpDyP) and three variants of the main distal residues (D143A, R232A and D143A/R232A). Arg-232 is found to be important for maintaining the heme distal architecture and essential to facilitate an alkaline transition. The latter is promoted in absence of Asp-143. Furthermore, the non-innocent effect of the buffer choice and addition of the cryoprotectant glycerol is shown. However, while unavoidable or indiscriminate experimental conditions are pitfalls, careful comparison of the effects of different exogenous molecules on the electronic structure and spin state of the heme iron contains information about the inherent flexibility of the heme pocket. The interplay between structural flexibility, key amino acids, pH, temperature, buffer and glycerol during in vitro spectroscopic studies is discussed with respect to the poor peroxidase activity of bacterial P-class DyPs.

Ionic strength and pH effect on the Fe(III)-imidazolate bond in the heme pocket of horseradish peroxidase: an EPR and UV-visible combined approach

The effects of chloride, dihydrogenphosphate and ionic strength on the spectroscopic properties of horseradish peroxidase in aqueous solution at pH=3.0 were investigated. A red-shift (lambda=408 nm) of the Soret band was observed in the presence of 40 mM chloride; 500 mM dihydrogenphosphate or chloride brought about a blue shift of the same band (lambda=370 nm). The EPR spectrum of the native enzyme at pH 3.0 was characterized by the presence of two additional absorption bands in the region around g=6, with respect to pH 6.5. Chloride addition resulted in the loss of these features and in a lower rhombicity of the signal. A unique EPR band at g=6.0 was obtained as a result of the interaction between HRP and dihydrogenphosphate, both in the absence and presence of 40 mM Cl-. We suggest that a synergistic effect of low pH, Cl- and ionic strength is responsible for dramatic modifications of the enzyme conformation consistent with the Fe(II)-His170 bond cleavage. Dihydrogenphosphate as well as high chloride concentrations are shown to display an unspecific effect, related to ionic strength. A mechanistic explanation for the acid transition of HRP, previously observed by Smulevich et al. [Biochemistry 36 (1997) 640] and interpreted as a pure pH effect, is proposed.

The formation of ferric haem during low-temperature photolysis of horseradish peroxidase Compound I

Illumination at low temperature of the peroxide compound of horseradish peroxidase (HRP-I) causes partial conversion of the haem electronic structure from a ferryl-porphyrin radical species into a low-spin ferric state. Magnetic-c.d. (m.c.d.) and e.p.r. spectral features of the photolysis product are almost identical with those of the alkaline form of ferric HRP, proposed on the basis of its near-i.r. m.c.d. spectrum to be a Fe(III)-OH species. The ferric product of HRP-I photolysis also contains free-radical e.p.r. signals. Conversion of HRP-I into the Fe(III)-OH species, which requires transfer of a proton and two electrons from the protein, is shown to be a two-step process.

Spectral properties of myeloperoxidase and its ligand complexes

The effects of ligands with various field strengths on the optical absorption spectrum of myeloperoxidase have been investigated. As is the case with other hemoproteins, the Soret peak in the optical absorption spectra at 77 K moves to longer wavelengths when strong-field ligands are present, whereas binding of such ligands as chloride and fluoride, which stabilize the high-spin state, shows the opposite effect. With a ligand of intermediate field strength, such as azide, the optical spectrum is not affected at room temperature, but lowering of the temperature results in the formation of the low-spin form of the enzyme. Similarly, in native myeloperoxidase a spin state equilibrium is found in which the low-spin state is favoured at high ionic strength and displays corresponding changes in the optical spectra. From the ligand- and the temperature-induced changes in the optical spectra of the ferric enzyme it is concluded that the band at 620-630 nm is an alpha band of the low-spin heme iron species, whereas the bands at 500 and 690 nm are probably 'charge-transfer' bands of the heme with the iron in the high-spin state.

The interaction of myeloperoxidase with ligands as studied by EPR

1. The reaction of myeloperoxidase with fluoride, chloride and azide has been studied by EPR. 2. Fluoride decreases the rhombicity of the high-spin heme signal of myeloperoxidase and the nuclear spin of the fluoride atom induces a splitting in g parallel of 35 G. This observation demonstrates that fluoride binds as an axial ligand to the heme iron of the enzyme. 3. Addition of chloride to the fluoride-treated enzyme increases the rhombicity of the high-spin heme signal and brings about a disappearance of the splitting at g parallel. The addition of azide to the fluoride-treated enzyme changes the spin state of the heme iron from a high-to a low-spin state (gx = 2.68, gy = 2.22 and gz = 1.80). 4. Upon addition of chloride or fluoride to low-spin azido-myeloperoxidase this compound is converted into the high-spin chlorido- or fluorido-myeloperoxidase. These observations demonstrate that these ligands compete for a binding site at or close to the heme iron of myeloperoxidase.

A spin label study of horseradish peroxidase

The topography of the active sites of native horseradish peroxidase and manganic horseradish peroxidase has been studied with the aid of a spin-labeled analog of benzhydroxamic acid (N-(1-oxyl-2,2,5,5-tetramethylpyrroline-3-carboxy)-p-aminobenzhydroxamic acid). The optical spectra of complexes between the spin-labeled analog of benzhydroxamic acid and Fe3+ or Mn3+ horseradish peroxidase resembled the spectra of the corresponding enzyme complexes with benzhydroxamic acid. Electron spin resonance (ESR) measurement indicated that at pH 7 the nitroxide moiety of the spin-labeled analog of benzhydroxamic acid became strongly immobilized when this label bound to either ferric or manganic horseradish peroxidase. The titration of horseradish peroxidase with the spin-labeled analog of benzhydroxamic acid revealed a single binding site with association constant Ka approximately 4.7 . 10(5) M-1. Since the interaction of ligands (e.g. F-, CN-) and H2O2 with horseradish peroxidase was found to displace the spin label, it was concluded that the spin label did not indeed bind to the active site of horseradish peroxidase. At alkaline pH values, the high spin iron of native horseradish peroxidase is converted to the low spin form and the binding of the spin-labeled analog of benzhydroxamic acid to horseradish peroxidase is completely inhibited. From the changes in the concentration of both bound and free spin label with pH, the pK value of the acid-alkali transition of horseradish peroxidase was found to be 10.5. The 2Tm value of the bound spin label varied inversely with temperature, reaching a value of 68.25 G at 0 degree C and 46.5 G at 52 degrees C. The dipolar interaction between the iron atom and the free radical accounted for a 12% decrease in the ESR signal intensity of the spin label bound to horseradish peroxidase. From this finding, the minimum distance between the iron atom and nitroxide group and hence a lower limit to the depth of the heme pocket of horseradish peroxidase was estimated to be 22 A.

Isolation procedure and some properties of myeloperoxidase from human leucocytes

1. A rapid isolation procedure with a high yield for pure myeloperoxidase (donor:H2O2 oxidoreductase, EC 1.11.1.7) from normal human leucocytes is described. The enzyme was solubilized from leucocytes with the detergent, cetyltrimethylammonium bromide, and purified to apparent homogeneity. The yield of the enzyme was 17% with an absorbance ratio A430nm/A280nm = 0.85. 2. The purified enzyme showed three isoenzyme bands after polyacrylamide gel electrophoresis; ultracentrifuge studies indicated one homogeneous band with a molecular weight of 144 000. After reduction of myeloperoxidase, sodium dodecyl sulfate gel electrophoresis resolved an intense band (63 000 daltons) and a weak band (81 000 daltons). 3. The carbohydrate content of the enzyme was at least 2.5%. Mannose, glucose and N-acetylglucosamine were present. The amino acid composition is reported. 4. The EPR spectrum exhibited a high-spin heme signal with rhombic symmetry (gx = 6.92, gy = 5.07 and gz = 1.95). Upon acidification this signal was converted into a signal with more axial symmetry (g perpendicular = 5.89). At high pH (9.5) the EPR spectrum of the enzyme only shows low-spin ferric heme resonances. The circular dichroism spectra of ferric and ferrous myeloperoxidase in the visible and ultraviolet region show maxima and minima in ellipticity.

Kinetic analysis of the acid-alkaline conversion of horseradish peroxidases

The nature of the acid-alkaline conversion of horseradish peroxidases was studied by measuring four rate constants in reactions, E + H+ (k1) in equilibrium (k2) EH+ and E + H2O (k3) in equilibrium (k4) EH+ + OH-, where EH+ and E denote the acid and alkaline forms of the enzymes. The values of k1, (k2 + k3), and k4 were obtained by measuring the relaxation rates of the acid leads to alkaline and alkaline leads to acid conversions by means of th pH jump method with a stopped-flow apparatus. The value of k3 could also be obtained by measuring the rate of reactions between hydrogen peroxide and peroxidases at alkaline pH. The measurements were conducted with four peroxidases having different pKa values: peroxidase A )pKa = 9.3), peroxidase C (pKa = 11.1), diacetyldeuteroperoxidase A (pKa = 7.7), and diacetyldeuteroperoxidase C (pKa = 9.1). The value of k1 was about 10(10) M-1 s-1 in the reaction of the four enzymes while k4 was quite different between the enzymes. The pKa was determined by k3 and k4 for the natural peroxidases and by k1 and k2 for the diacetyldeuteroperoxidases. The mechanism of the acid-alkaline conversion was discussed in comparison with that of metmyoglobin.

Visible absorption spectra of quantum mixed-spin ferric heme proteins

Recently, it has been shown that the magnetic data for Chromatium ferricytochrome c' at pH 7 are consistent with quantum mechanically (as distinguished from thermally) mixed mid-spin (S = 3/2) and high-spin (S = 5/2) heme. Visible absorption spectra of the protein measured at 77 degrees K and 293 degrees K, pH 7, show peaks at 400, 490, and 632 nm. The observation of a 630 nm band in quantum mixed-spin heme spectra, and the spin state-dependence of the band intensity, are discussed in the context of the iron-ligand structure for quantum mixed-spin heme inferred from magnetic data.

Laser Raman spectra of oxidized hydroperoxidases

Resonance Raman spectra of oxidized hydroperoxidases are examined for shifts in the structure-sensitive, anomalously polarized bands; these are found, respectively, at 1576, 1567 and 1570 cm-1 in the high-spin resting enzymes: horse radish peroxidase, horse blood catalase, and cytochrome c peroxidase. In compound II of horse radish peroxidase and horse blood catalase, and in the enzyme-substrate complex of cytochrome c peroxidase, this band appears at 1587-1590 cm-1 and indicates the iron atom is now in-plane with the porphyrin ring. Weak Raman scattering found with horse radish peroxidase I is consistant with a porphyrin eta-cation radical formulation.

The spin 3/2 state and quantum spin mixtures in haem proteins

The magnetic state of iron in haem proteins has long been recognized as a convenient indicator of chemical coordination as well as more subtle biochemical properties. In the trivalent case in particular, there are three magnetically distinct configurations of the five 3delectrons, yielding spin states ofS= 1/2, 3/2 and 5/2. The first and third are extremely familiar, and often lie close enough in energy so that athermalmixture of low-spinS= 1/2 and high-spinS= 5/2 states exists in an ensemble of molecules. Selection rules for common perturbations (ΔS= ∘, ± 1 for the spin-orbit interaction and ΔS= ∘ for the electronic Zeeman interaction) ensure thatquantummixtures, in which the wave function is a true combination ofS= 1/2 andS= 5/2 components, are not observed. The mid-spin,S= 3/2, state, and allowed 5/2–3/2 and 1/2–3/2 quantum mixtures including it, are much less well known. These have only rarely been invoked in recent years as an explanation for experimental haem protein magnetic data.

Heme-linked proton dissociation of carbon monoxide complexes of myoglobin and peroxidase

It was found from spectrophotometric titration and proton balance measurement that the pKa value of a heme-linked protonation group of horseradish ferro-peroxidase C (donor:H2O2 oxidoreductase, EC 1.11.1.7) shifted from 7.25 to 8.25 upon combination with CO. The spectrophotometric titration experiment with myoglobin also revealed the presence of a heme-linked protonation group, the pKa value being 5.57 in myoglobin and 5.67 in the CO-myoglobin complex. It was concluded that the distinct shift of the pKa value in the case of peroxidase was attributable to the presence of a hydrogen bond between the sixth ligand and the distal base. The difference in the strength of such hydrogen bonding between peroxidase and myoglobin was discussed.